drm/i915: Unify legacy/execlists emission of MI_BATCHBUFFER_START

Both the ->dispatch_execbuffer and ->emit_bb_start callbacks do exactly
the same thing, add MI_BATCHBUFFER_START to the request's ringbuffer -
we need only one vfunc.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1469432687-22756-20-git-send-email-chris@chris-wilson.co.uk
Link: http://patchwork.freedesktop.org/patch/msgid/1470174640-18242-10-git-send-email-chris@chris-wilson.co.uk
diff --git a/drivers/gpu/drm/i915/intel_ringbuffer.h b/drivers/gpu/drm/i915/intel_ringbuffer.h
index 76d0495..45ba29c 100644
--- a/drivers/gpu/drm/i915/intel_ringbuffer.h
+++ b/drivers/gpu/drm/i915/intel_ringbuffer.h
@@ -214,12 +214,6 @@
 	 * monotonic, even if not coherent.
 	 */
 	void		(*irq_seqno_barrier)(struct intel_engine_cs *engine);
-	int		(*dispatch_execbuffer)(struct drm_i915_gem_request *req,
-					       u64 offset, u32 length,
-					       unsigned dispatch_flags);
-#define I915_DISPATCH_SECURE 0x1
-#define I915_DISPATCH_PINNED 0x2
-#define I915_DISPATCH_RS     0x4
 	void		(*cleanup)(struct intel_engine_cs *engine);
 
 	/* GEN8 signal/wait table - never trust comments!
@@ -297,7 +291,11 @@
 #define EMIT_FLUSH	BIT(1)
 #define EMIT_BARRIER	(EMIT_INVALIDATE | EMIT_FLUSH)
 	int		(*emit_bb_start)(struct drm_i915_gem_request *req,
-					 u64 offset, unsigned dispatch_flags);
+					 u64 offset, u32 length,
+					 unsigned int dispatch_flags);
+#define I915_DISPATCH_SECURE 0x1
+#define I915_DISPATCH_PINNED 0x2
+#define I915_DISPATCH_RS     0x4
 
 	/**
 	 * List of objects currently involved in rendering from the